Phycocyanin (PC) is a blue pigmented billiprotein, a chromophore produced in prokaryotic cyanobacteria as well as certain eukaryotes such as the rhodophytes, cryptomonads and glaucocystophytes. PC is increasingly being exploited as a natural food colouring, replacing the synthetic dye Brilliant Blue FCF that has been associated with health problems; PC is particularly suited to this use because of its high solubility in water and stability over a large pH range [1]. In addition, PC is used in the nutraceutical, pharmaceutical and cosmeceutical industries at higher purities for its anti-oxidant and anti-inflammatory properties, together with other associated health benefits [2-4]. PC in its more crude form is also used as an additive to animal feeds to enhance the colour of ornamental fish and birds. At its highest quality and purity PC is used in laboratory assay kits for its fluorescent properties. There is also early but ongoing research into the therapeutic properties of PC for medical use [5]. The PC market is in its infancy.
In cyanobacteria, PC is present in the thylakoid membranecomplexed with the other biliproteins including phycoerythrin (PE) and allophycocyanin (AP or APC) which together function as a light-harvesting apparatus known as the phycobillisome [6]. The phycobillisome absorbs specific wavelengths of light that cannot be utilized by chlorophyll, thereby increasing the efficiency of photosynthesis [7]. PC absorbs maximally at 610-620 nm with PE (540-570 nm) and APC (650-655 nm) [6].
Cyanobacteria are widely used in aquaculture for PC production with the eukaryotes showing potential for future exploitation. Among the cyanobacteria the genus Arthrospira (formerly known as Spirulina and still commercially known as ‘Spirulina’) is the most commonly cultured genus; however, PC has been extracted from other genera such as Aphanizomenon and Anabaena. The main species in culture are Arthrospira platensis and A. maxima. These are both filamentous cyanobacteria with spiral-shaped filaments or trichomes.
In addition to its high PC content, spirulina also contains high amounts of other nutraceuticals such as vitamins and PUFAs and is high in single cell protein; as a result, PC is becoming of increasing commercial interest in the West [1]. In the East and Africa however, Spirulina has been used as a food for many centuries [9]. Spirulina biomass is a salable product alone, however pure phycocyanin, depending on purity has a considerably higher market price.
As water molecules absorb in the far red region of light, limitations in this wavelength for photosynthesis occur in the natural algae environment [6]. Light scattering of shorter wavelengths also occurs by suspended material resulting in the provision bias of blue-green wavelength light to algae in nature. Therefore environmental factors determine light availability and algae can adapt to utilize quality and quantity of light available.
Some cyanobacteria containing PE and PC exhibit a phenomenon called complementary chromatic adaptation where PC:PE ratio is altered in response to different light regimes by modulating synthesis [10,11]. Recent research has shown that A. platensis can be manipulated in the presence of certain wavelengths of light to increase production of PC.
By using light filters Walter et al. (2011) [12] demonstrated increased PC purity under red light (600-700 nm). Earlier research by Wang et al. also found A. platensis biomass productivity was higher culturing under red light [13]. The calculations by Wang et al. demonstrated that the use of red light would be economically beneficial to photoautotrophic production, as energy to biomass conversion is more efficient. Farges, 2009 [19] also modeled growth of A. platensis under polychromatic and monochromatic light sources, demonstrating mathematical increases in culturing efficiency under red 620 nm LEDs through decreased electrical energy power consumption with maintained and comparable growth rates; however monochromatic red LED light (620 nm) was shown to decrease the PC concentration of A. platensis by 2-fold, compared with white/polychromatic and red+blue polychromatic LEDs. These studies have demonstrate that culturing under different wavelengths of light can effect the PC concentration and purity in A. platensis cultures, however tests have not been conducted using wavelengths above that of normal red LEDs.
There is therefore a need in the art for improved and less costly methods for synthesis of phycocyanins.